This invention relates to a hydraulic brake system for automotive vehicles with a master cylinder and with a hydraulic power booster connected upstream of the master cylinder, in which a pressure medium pump which may be driven by an electric motor is used for providing an auxiliary hydraulic energy the drive of which may be switched on by a pressure accumulator which in the unbraked operation of the automotive vehicle is permanently kept on a pressure level sufficient for an initial actuation of the brake and which is charged via a pressure control valve in which a valve passage is operable by a hydraulic pressure in the booster chamber of the power booster such as to ensure that upon pressurization of the booster chamber a connection is locked between the pressure accumulator and the power booster, according to patent (patent application No. P 33 15 731.6).
The subject matter of the aforementioned German patent application is a hydraulic brake system where a pressure medium pump is provided which may be driven by an electric motor. The drive of said pump may be switched on by a brake pedal contact just as well as it may be operated in dependence on the charge of a pressure accumulator. In the unbraked operation of the automotive vehicle the pressure medium pump will be running until a predeterminable charge will have been established in a hydraulic pressure accumulator which will provide a sufficient auxiliary hydraulic energy in the initial phase of braking. This charge having been established, the drive of the pressure medium pump will be switched off.
The pressure accumulator will be charged via a pressure control valve which, on the one hand, has a free hydraulic connection between the pressure port of the pressure medium pump and the inlet of the hydraulic power booster and which, on the other hand, disposes over a valve passage which may be closed by a hydraulic pressure in the pressure chamber of the power booster. Said valve passage is normally open and will come into a closing position as soon as, upon an actuation of the brake, a hydraulic pressure will enter the pressure chamber of the power booster.
In the initial phase of braking the pressure medium pump will start operating when the electric brake pedal contact closes. Due to the inertia of the pressure medium pump, however, there will not be available the full delivery pressure immediately after the start of the pressure medium pump. In this phase the pressure accumulator will be discharged towards the hydraulic power booster via the still open valve passage of the pressure control valve. Thus, a sufficient auxiliary hydraulic energy will also be available in the initial operating phase of the pressure medium pump.
The pressure in the pressure chamber of the hydraulic power booster having reached a predeterminable level, the valve passage of the pressure control valve will come into a closing position in which a connection will be interrupted between the delivery side of the pressure medium pump and the inlet of the pressure accumulator. Thus, the volume delivered by the pressure medium pump will exclusively be supplied to the inlet of the hydraulic power booster. In this operating condition, it will be prevented that the pressure accumulator will be charged anew by the pressure medium pump.
In the described brake system it is necessary that the pressure port of the hydraulic power booster be locked in the unbraked operation and that the booster chamber hydraulically communicate with an unpressurized supply reservoir so as to prevent any pressure from becoming effective at the control port of the pressure control valve. This is generally the case in hydraulic power boosters with a so-called closed center. In certain applications, however, it may be advantageous to use a hydraulic power booster with a so-called open center (throttle-type booster). In the non-operating condition, however, such power boosters will have a free passage from the pressure port to the booster chamber. Upon the actuation of the brake a volume stream supplied by a circulation pump will be throttled in accordance with the actuating force so that there will result a corresponding hydraulic pressure in the pressure chamber. The idea forming the basis of the brake system according to patent application No. P 33 15 731.6 thus cannot be applied directly to such brake systems with throttle-type boosters.
It is thus an object of the present invention to provide a hydraulic brake system of the type referred to above in which a throttle-type booster is used as hydraulic power booster.